This invention relates to electromagnetic discharge apparatus driven by high frequency power sources and more particularly to electrodeless light sources having an improved means for automatic starting.
Electrodeless light sources which operate by coupling high frequency power to an arc discharge in an electrodeless lamp have been developed. These light sources typically include a high frequency power source connected to a termination fixture with an inner conductor and an outer conductor disposed around the inner conductor. The electrodeless lamp is positioned at the end of the inner conductor. High frequency power is coupled to a light emitting electromagnetic discharge in the electrodeless lamp. A portion of the termination fixture passes radiation at visible light frequencies, thus permitting use of the apparatus as a light source.
The electrodeless lamp in its operating condition represents a relatively low impedance of approximately a few hundred ohms. However, in the off state the impedance of the lamp is high. Since the termination fixture is designed to effect an impedance match to the operating impedance of the lamp, thus obtaining maximum transfer of power from the source to the arc discharge, there exists in the off state a mismatch between the lamp and the high frequency power source. This off-state mismatch creates a problem in starting a discharge when power is first applied to the light source. Because of the mismatch, insufficient forward directed power is delivered to the lamp to cause starting. A tuning element located in the termination fixture is used for starting in U.S. Pat. No. 4,002,944 issued Jan. 11, 1977 to McNeill et al. A resonant condition is created which causes a strong electric field to initiate breakdown and excitation of the fill material within the lamp.
The use of ultraviolet light sources to start the discharge in electrodeless lamps is described in U.S. Pat. No. 3,997,816 issued Dec. 14, 1976 to Haugsjaa et al. An ultraviolet source is placed near the electrodeless lamp and provides free photoelectrons which, in combination with a high frequency electric field from the power source, induce starting of the electrodeless lamp. Either a glow lamp or a spark generating device is located in the space between the inner and outer conductors of the termination fixture. The glow lamp is series with a bimetallic switch is connected across the conductors of the termination fixture. When the electrodeless lamp heats up, the bimetallic switch opens and the glow lamp is turned off. A variation on this method of starting an electrodeless lamp is shown in U.S. Pat. No. 4,041,352 issued Aug. 9, 1977 to McNeill et al. The ultraviolet source is connected in series with the high frequency power source thus reducing the voltage supplied to the high frequency power source. After starting the electrodeless lamp, a bimetallic switch shorts out the ultraviolet source and provides full voltage to the high frequency power source. Another method for starting an electrodeless lamp is shown in U.S. Pat. No. 4,053,814 issued Oct. 11, 1977 to Regan et al. The ultraviolet source is connected in series with the high frequency power source. A control circuit utilizing a photosensitive resistor reduces the DC voltage to the ultraviolet source in a continuously varying manner as the electrodeless lamp increases its light output.
Lower DC input voltage to the high frequency power source during lamp starting as shown in the two previously mentioned patents results in lower power delivered to the electrodeless lamp, thereby reducing the voltage standing waves caused by a mismatched load. Excessive voltage standing waves could result in damage to the high frequency power source. A solid state microwave power source for use in an electrodeless lamp is described in U.S. Pat. No. 4,070,603 issued Jan. 24, 1978 to Regan et al. Better matching to the electrodeless lamp during starting is a feature of the power source. Thus, the possibility of damage to the power source during starting is reduced.
While the above-described methods for starting electrodeless light sources give generally satisfactory results, such techniques have certain disadvantages. Some of these disadvantages are circuit complexity which in turn reduces reliability, mechanical complexity of the termination fixture and the requirement for several manual operations to effectuate starting.
The following U.S. patents relate generally to electrodeless light sources and may be of interest.
______________________________________ U.S. Pat. No. Patentee Issue Date ______________________________________ 3,942,058 Haugsjaa et al Mar. 2, 1976 3,942,068 Haugsjaa et al Mar. 2, 1976 3,943,401 Haugsjaa et al Mar. 9, 1976 3,943,402 Haugsjaa et al Mar. 9, 1976 3,943,403 Haugsjaa et al Mar. 9, 1976 3,943,404 McNeill et al Mar. 9, 1976 3,993,927 Haugsjaa et al Nov. 23, 1976 3,995,195 Haugsjaa et al Nov. 30, 1976 4,001,631 McNeill et al Jan. 4, 1977 4,001,632 Haugsjaa et al Jan. 4, 1977 4,010,400 Hollister Mar. 1, 1977 4,063,132 Proud et al Dec. 13, 1977 4,065,701 Haugsjaa et al Dec. 27, 1977 ______________________________________